The present invention relates to an optical recording medium such as optical disks having a half-transmissive film.
As demands for an optical recording medium, such as optical disks, of higher density and smaller size increase, optical disks such as digital video disks (DVD) that allow two types of signals to be read from one side of the disk have been developed and are now available in the market. Such a one-sided disk has a double layer structure of a total-reflection film and a half-transmissive film.
Below, one example of a conventional one-sided, read-only dual layer DVD is described with reference to FIGS. 9A and 9B. FIG. 9A illustrates an actual construction of the disk while FIG. 9B is a conceptualized view of same. A first resin substrate 21 is made of, for example, polycarbonate (PC) or polymethyl methacrylate (PMMA). A half-transmissive film 22 of Au is deposited on a backside of the resin substrate 21 by sputtering or the like. A second resin substrate 23 is made of PC, PMMA or the like, and a total-reflection film 24 of Al deposited on a front surface of the second resin substrate 23 by means of sputtering or the like. These resin substrates 21, 23, on which respective layers have been formed, are bonded together with a transparent adhesive 25 made of a UV-curable photo-polymer or the like, thereby forming a one-sided double layer disk. A laser beam 26 of 650nm wavelength is shown and a a reflected beam 27 are shown. The laser beam 26 passes through the resin substrate 21 and is reflected by the Au half-transmissive film 22. A reflected beam 28 is produced when the laser beam 26 passes through the resin substrate 21, the Au half-transmissive film 22, the adhesive layer 25, and is reflected by the Al total-reflection film 24.
The one-sided, read-only double layer disk constructed as described above works as explained below. In this double layer disk, the reflectivity of the Au half-transmissive film 22 is set to be equal to the reflectivity of the Al total-reflection film 24 relative to the wavelength of the laser light 26 for reproduction of data, whereby reproduction signals can be read equally from the two layers. For this reason, the reflective layer on the side on which the laser beam 26 is incident must be a half-transmissive film, and its reflectivity is generally set at 25 to 30% so as to equalize the reflected light 27 and 28.
The Au half-transmissive film 22 in the prior art exhibits the reflectivity characteristics required thereof, having a complex index of refraction of 0.19 and absorption coefficient of 3.5. Half-transmissive films are also demanded to be reliable not to change in outer appearance or reflectivity, due oxidization and corrosion, even in an environment under severe temperature and humidity conditions: The above described Au half-transmissive film 22 is also satisfactory in this regard, as it is made of a material which is remarkably resistant to oxidization and corrosion.
However, in the one-sided double layer disk described above, the use of Au for the half-transmissive film 22, which costs 20 to 30 times more than other metal or dielectric materials deposited by means of sputtering, raises the cost of disks considerably. If, by way of example, the price of Au is 1400 yen per gram, the price of the half-transmissive film 2 per one disk is 3.5 yen.